1. Field of the Invention
The present invention relates to a lighting control circuit for a vehicle lighting fixture and in particular to a lighting control circuit designed to control lighting of a semiconductor light source including a semiconductor light-emitting device.
2. Description of the Related Art
In the related art, a vehicle lighting fixture is known that uses a semiconductor light-emitting device such as an LED (Light Emitting Diode). This type of vehicle lighting fixture mounts a lighting control circuit for controlling lighting of an LED.
A lighting control circuit has been proposed that is, in a configuration where a plurality of LEDs are serially connected to each other to form a light source unit and a plurality of light source units are connected to each other in parallel, connected to each of both ends of the plurality of light source units connected in parallel (refer to JP-A-2004-134147, pages 3-6, FIG. 1). In this configuration, the lighting control circuit feeds the same current to all LEDs of each light source unit. A resistor is inserted serially to each light source unit. When a voltage across the resistor has dropped, for example the current is not supplied to the resistor upon a wire break in any LED of the light source unit and the voltage across the resistor drops to 0V, a wire break in any LED is assumed, and the output voltage of a switching regulator including the lighting control circuit is lowered. According to this lighting control circuit, when wire break in any LED of each light source unit occurs, the output voltage of the switching regulator is lowered, which prevents the output voltage of the switching regulator from reaching overvoltage.
JP-A-2004-134147 (pages 3-6 and FIG. 1) is referred to as a related art.
There are severals cases of wire breaks in an LED. When a wire (lead wire) connected to an LED is broken, it is possible to detect the wire break in the LED by detecting that a current does not flow in the LED. When a wire break occurs inside the LED, an attempt to detect the wire break could fail. For example, in case a semiconductor chip serving as a light-emitting device and a Zenor diode connected in parallel with the semiconductor chip are housed in an LED package, a current may keep flowing through the Zenor diode irrespective of a wire break in the semiconductor chip or wire bonding. Thus, the wire break may not be detected.
More specifically, on a wire break in the semiconductor chip in an LED package, a current does not flow through the semiconductor chip. This reduces the load on the power supply circuit and increases a voltage across the LED package. When the voltage rises and exceeds the forward voltage of the Zenor diode thus reaching the Zenor voltage of the Zenor diode, a current starts to flow through the Zenor diode and the voltage across the LED package changes from the forward voltage of the LED to a Zenor voltage. When N LED packages are serially connected, the voltage across the entire package changes from forward voltage×N to (forward voltage×(N−1)+Zenor voltage). When a current flows through the Zenor diode, the power supply circuit executes control to feed the same current to the LEDs. This applies a high voltage across the LED package where a wire break in the semiconductor chip has taken place. The voltages rises so that power consumption in the Zenor diode increases, which could result in thermal breakdown of the Zenor diode, thus lowering the Zenor voltage below the forward voltage of the LED in a thermally stable fashion.
In the case where a wire break has occurred in an LED package using a Zenor diode as an electrostatic protection rather than protection against a wire break, a current continues to flow via the Zenor diode. Thus, it may not be possible to reliably detect a wire break in an LED only by monitoring a voltage drop in a resistor serially connected to a light source unit.
When an LED without a Zenor diode for electrostatic protection or an LED using a capacitor instead of a Zenor diode as a semiconductor light source, it is necessary to consider a “short” and a “short having a certain impedance” as fault modes of LED. For example, even when one of the LEDs serially connected shorts, a current flows in each LED. Thus, it is necessary to monitor the voltage applied to a plurality of LEDs from a switching regulator to detect an error that accompanies a short-circuit fault in an LED. For example, it is possible to use a configuration where the output voltage of the switching regulator is compared with a set voltage, and when the output voltage of the switching regulator has dropped below the set voltage, a short-circuit fault in any LED is detected. Considering variations in the LED voltage, the configuration where the output voltage of the switching regulator is compared with the set voltage may fail to detect a short-circuit fault in any one of serially connected LEDs.
For example, assuming a voltage drop per LED, that is, a case where eight LEDs each having a forward voltage Vf=8V are connected in series, the output voltage of the switching regulator is 64V. While Vf=8V of the LED is assumed, the voltage is subjected to variations. Causes of such variations include “the VI characteristic of an LED”, “the temperature characteristic of an LED” and “individual differences of LEDs”.
The VI characteristic is such that Vf becomes larger as the current flowing in an LED (If) becomes larger. The temperature characteristic is such that Vf becomes smaller as the temperature of an LED becomes higher. Considering variations in Vf as Vf=7V to 9V, the output power range permitted as an output voltage of the regulator is 56V to 72V. In this practice, in a case where one LED goes faulty due to a short-circuit for some reason, assuming that Vf of 8V of the faulty LED becomes 0V, a switching regulator whose normal output voltage is 72V provides an output voltage of 64V irrespective of the short-circuit fault. The output voltage is within the output voltage range permitted for a switching regulator, so that only monitoring the output voltage of a switching regulator cannot detect a short-circuit fault. In other words, considering variations in the LED voltage, simply monitoring the absolute value of the output voltage of a switching regulator may not detect a short-circuit fault on one of the LEDS.
When one of the LEDs goes faulty due to a short-circuit, for example, when one LED does not illuminate due to a short-circuit fault, the lighting fixture is emitting light although the desired light distribution is not satisfied. The driver does not notice a fault and he/she may continue driving.
When a short-circuit fault detection circuit is provided for each LED, it is possible to detect a short-circuit fault in an LED without considering variations in the LED voltage. In this approach, however, the number of short-circuit fault detection circuits increases and wiring becomes thicker as the number of LEDs becomes greater, which complicates the circuit configuration.